The present invention relates to a chemical metallization process for metallizing a substrate of the type in which a radiation sensitive sensitizer is applied to the substrate and is then exposed to light, after which the sensitizer is treated with a salt solution of a catalytically effective metal. In particular, the invention relates to a process for manufacturing printed circuits, also called circuit boards, which are used in the electrical industry.
The production of circuit boards for use in electronics is presently still primarily handled according to the so-called subtractive teghnique. The basic material is a plastic substrate material which is coated with copper over its entire surface area and on which conductors are produced by etching away the surplus copper, which may amount to 80% of the total copper employed. Disadvantageous underetching of the metallic conductor paths can be overcome only partially by compensation in the conductor layout and/or by providing protection on the sides of the conductor paths.
To mark the surface pattern to be etched, organic photoresist layers are required and sometimes also additional metalresist coatings. For the last few years, the so-called semiadditive technique has been available. This technique requires only two thirds of the process steps of the subtractive technique and produces less copper waste, but still requires masking and etching processes.
These drawbacks are eliminated in the so-called fully additive technique, particularly if lacquer-free photoadditive process is used, a process which is most economical with respect to the consumption of raw materials and is least damaging to the environment. In a simplified manner, the photoadditive technique can be subdivided into a process scheme which includes the basic material and its pretreatment, image transfer and finally selective activation as well as subsequent metallization.
An important step, for example, in the production of circuit boards, is the transfer of the conductor pattern from a master to the circuit board substrate material. In the photoadditive technique, this requires the application of a sensitizer to the surface of the circuit board, with such sensitizer being activated or deactivated at selected locations by selective exposure to light. Noble metal nuclei can be precipitated on the activated regions from a noble metal salt solution which can then be metallized in a metallization bath operating without external current.
The activated sensitizer plays a key role in the various photoadditive processes. For example, DE-AS [Federal Republic of Germany Published Application] No. 1,917,474 discloses, inter alia, a sensitizer containing a tin(II) salt. This tin(II) salt is converted by UV light into tin(IV) which then is no longer able to reduce, for example, palladium chloride in aqueous solution, to palladium nuclei.
The drawback of these sensitizers containing tin(II) is their oxidation sensitivity with respect to oxygen in the air. For example, solutions of tin(II) salts are oxidized to tin(IV) salts in air and these hydrolize in aqueous solutions and result in multinucleus tin oxide hydrates which are deposited at the bottom of the vessel as a gel-like colorless precipitate. The tin(II) content of such a solution thus continuously decreases over time. Without adapting the exposure times and/or keeping constant the tin(II) concentration, underexposures and/or reflections between an exposure mask and base material would have an annoying influence, particularly in the assembly of the thinnest finest conductors. Titanium and lead salts behave in a manner similar to the tin salts.
DE-AS No. 1,917,474 additionally mentions an iron(III) oxalate photopromoter whose reaction under the influence of actinic radiation is stated to take place as follows: EQU Fe.sub.2 (C.sub.2 O.sub.4).sub.3 .fwdarw.2Fe.sup.3+ +3(C.sub.2 O.sub.4).sup.2- EQU (C.sub.2 O.sub.4).sup.2- .fwdarw.2 CO.sub.2 +2 e.sup.- EQU 2 Fe.sup.3+ +2 e.sup.- .fwdarw.2 Fe.sup.2+
The resulting iron with oxidation number +2 is then able to precipitate, for example, palladium seeds, or nuclei, from a palladium chloride solution according to the following formula: EQU 2 Fe.sup.2+ +Pd.sup.2+ .fwdarw.2 Fe.sup.3+ +Pd.sup.0.
One drawback of this photopromoter is the presence of iron with oxidation number +2 with which the iron(III) salts are in equilibrium. The iron(II) present without exposure to light effects nucleus formation in the sense of the above equation and may be the cause of an unintended metal precipitation at locations on a surface not intended for such precipitation. This drawback can be avoided by the addition of an oxidation agent, e.g. nitric acid, in trace quantities so as to convert the undesirable iron(II) into iron(III). If the oxidation agent, e.g. nitric acid, is given in excess amounts, which is possible, the drawback occurs that the Fe.sup.2+ ions produced by exposure to light are immediately converted to Fe.sup.3+ ions so that no nucleation is possible. Such process is economically unfeasible.
DE-OS [Federal Republic of Germany Laid-Open Application] No. 2,518,520 discloses the use of colloidal solutions of oxyhydrates of palladium, indium, cerium, nickel, manganese, uranium, molybdenum and tungsten as photosensitive sensitizers. One drawback of these colloidal solutions is that the most varied events may cause flocculation of the colloidally dissolved components. The probability of flocculation increases with the number and concentration of foreign substances, e.g. grains of dust.